1. Field of the Invention
The present invention relates to a vacuum pressure control system capable of controlling vacuum pressure in vacuum vessels at a prescribed value, the vacuum vessels being usually used in semiconductor producing processes.
2. Description of Related Art
In a conventional vacuum pressure control system, which is shown in FIG. 16, usually a vacuum chamber 11 serving as a vacuum vessel is provided with an inlet 13 and an outlet 14, in which wafers 15 to be processed are placed in tires. The inlet 13 is connected with a source of process gas and a source of nitrogen gas which is used for purging the inside 12 of the vacuum chamber 11. The outlet 14 is connected to an inlet port of a pilot type opening and closing valve 20 which consists of a bellows poppet valve. An outlet port of the pilot valve 20 is connected via an opening proportional valve 23 to a vacuum pump 19. The opening proportional valve 23 consists of a butterfly type of opening adjustment valve.
The proportional valve 23 is formed of a circular plate arranged within a pipe extending between the pilot valve 20 and the vacuum pump 19 which has an approximately same diameter as the inner diameter of the pipe and is rotatably about a support shaft provided along the center line of the circular plate. The support shaft is rotated and stopped by a step motor. The opening degree of the proportional valve 23 is dependent on the stop position of the step motor. Usually, measurement of the vacuum pressure inside of the vacuum chamber 11 is made by a pressure sensor 17 and its measured values are fed back to control the stop position of the step motor of the opening proportional valve 23 so as to equalize a vacuum pressure to be measured with a prescribed value of vacuum pressure.
During a producing process, the vacuum chamber 11 is supplied with process gas. In the conventional vacuum pressure control system, the proportional valve 23 is controlled to increase its opening degree when the vacuum pressure value become higher toward the atmospheric pressure than a desired value, thereby allowing the vacuum pump 19 to suck a larger amount of flow of the gas. The proportional valve 23 is, to the contrary, controlled to decrease its opening when the vacuum pressure value become lower toward the absolute value than the desired value, thereby allowing the vacuum pump 19 to suck a lesser amount of flow of the gas.
The butterfly type proportional valve like the aforesaid opening proportional valve 23 can not effect a complete shutoff due to its construction. In trying to completely shut off the pipe with the butterfly type proportional valve, the butterfly valve is usually equipped with an O-ring and the like around the valve to contact with the inner wall of the pipe. In semiconductor producing apparatuses, however, products produced from process gas would be deposited on the surface of the O-ring and make it impossible for the butterfly valve to fully shut off the pipe. It is also impossible to apply a torque load sufficient to the O-ring in order to effect the complete shutoff. The conventional vacuum pressure control system therefore needs an emergent shutoff valve like the pilot type opening and closing valve 20 to be connected to the proportional valve 23 in series.
The shutoff valve needs to have, in addition to the full vacuum shutoff function, an emergency shutoff function to shut off flow of gas as soon as power to the apparatus is cut off. A cylinder type pilot valve is used for the pilot valve 20 accordingly.
In creating a vacuum in the vacuum chamber 11, if the opening of the proportional valve 23 is set larger while a large amount of process gas remains in the vacuum chamber 11, a great volume of the process gas is sucked from the vacuum chamber 11 for a short time, inducing gas current inside thereof and thereby causing a problem of flying up particles which have adhered to the inner wall of the vacuum chamber 11.
Suction of all the particles flied up can not easily be effected. When a large amount of process gas remains in the vacuum chamber 11, usually, control of the suction amount of process gas is effected by increasing little by little at first the opening of the proportional valve 23. To enable this, the proportional valve 23 must be kept stable in a small opening.
However, within a small opening range, the butterfly type proportional valve would largely change its opening by a slight rotation angle due to its construction. It is therefore difficult to set the proportional valve 23 stable with its opening small and slowly outgas from the vacuum chamber 11 by means of the vacuum pump 19.
To solve the problem, in the conventional system, a bypass valve 22 and a shutoff valve 21 are connected in a line. The bypass valve 22 is formed to have a small opening. Closing the pilot valve 20, opening the proportional valve 23 and then opening the shutoff valve 21, the vacuum chamber 11 can be connected to the vacuum pump 19 through a small opening to make it possible to suck slowly the process gas remaining in the vacuum chamber 11.
The control of the suction is effected by a vacuum control device (not illustrated) based on the outputs measured by the pressure sensor 17 to measure the vacuum pressure of the outlet 14. A shutoff valve 16 used for the maintenance of the pressure sensor 17 is further disposed between the outlet port 14 and the pressure sensor 17.
The conventional vacuum pressure control system, however, has the following problems.
(1) A combination of the opening proportional valve 23 consisting of a butterfly type proportional valve and the bypass valve 22 can not effect the precise control of vacuum pressure in a wide range from a low vacuum region including the vacuum pressure near the atmospheric pressure (ex. several Torr), a medium region, to a high region (ex. several mmTorr). In particular, the butterfly valve can not allow a little amount of flow of gas to pass therethrough. The conventional system, even if using the bypass valve, can not cope with the necessity of slight variations in vacuum pressure because the amount of flow of gas is kept constant by the bypass valve.
(2) Besides the proportional valve 23, it needs the pilot valve 20, the bypass valve 22 and the shutoff valve 21, having many joints in a pipe line. This may cause the mixture of particles and others from the joints into the pipe line. The combination of the valves makes the size of a whole apparatus larger and also its using process more complex. This is a problem for the equipment to be used in the semiconductor producing process which requires compactness to each equipment. In addition, there is an issue of cost up in the above equipment.
(3) It is conceivable to utilize a poppet valve for a means of resolving the above problems. This means is that a step motor or a servo motor controls the position of a flat valve to come into contact with and separate from a seat valve. This realizes an integrated component of the proportional valve 23, the bypass valve 22 and the shutoff valve 21. It is however difficult to provide the emergent shutoff function to the poppet valve to be driven by the step motor or the servo motor. Because the step motor or the servo motor stops at the time of interruption of electric service and thus can not close the poppet valve in an emergency. When the valve needs to be closed at a high speed, there is also a problem of it taking about ten seconds for the valve to fully close since the rotation of a step motor can not be speeded up due to the necessity of generating the torque to drive the poppet valve.